Fire is the rapid oxidation of a material in the exothermic chemical process of combustion, releasing heat, light, and various reaction products.
Fires start when a flammable and/or a combustible material, in combination with a sufficient quantity of an oxidizer such as oxygen gas or another oxygen-rich compound (though non-oxygen oxidizers exist that can replace oxygen), is exposed to a source of heat or ambient temperature above the flash point for the fuel/oxidizer mix, and is able to sustain a rate of rapid oxidation that produces a chain reaction.
A fire extinguisher is an active fire protection device used to extinguish or control small fires, often in emergency situations, often being the first line of defense when a fire breaks out.
Typically, a fire extinguisher consists of a hand-held cylindrical pressure vessel containing an agent which can be discharged to extinguish a fire, whereas two main types of fire extinguishers are usually used: stored pressure and cartridge-operated.
In stored pressure units, the expellant is stored in the same chamber as the fire-extinguishing agent itself. Depending on the type of agent used, different propellants may be used. With dry chemical extinguishers, nitrogen is typically used, whereas water and foam extinguishers typically use air.
Cartridge-operated extinguishers contain the expellant gas in a separate cartridge that is punctured prior to discharge, exposing the propellant to the extinguishing agent. This type is not as common as the stored pressure units, and is used primarily in areas such as industrial facilities, where they receive higher-than-average use. They have the advantage of simple and prompt recharge, allowing an operator to discharge the extinguisher, recharge it, and return to the fire in a reasonable amount of time. Unlike stored pressure types, these extinguishers use compressed carbon dioxide instead of nitrogen, although nitrogen cartridges are used on low temperature (−60 rated) models. Cartridge operated extinguishers are available in dry chemical and dry powder types and also in water, wetting agent, foam, dry chemical and dry powder types.